关键词: 表面增强拉曼光谱, 铑, 粗糙, 一氧化碳, 吡啶

Abstract: Raman Spectroscopy is a powerful technique in characterizing the molecular structure at the molecular level. However, only after the discovery of surface enhanced Raman scattering (SERS) effect, has it become one of the most widely used technique in surface sciences. The limitation that only Ag, Au and Cu can produce prominent SERS signal of practical significance, greatly hinders the application of this technique. Recently our group has made great progress in extending SERS to pure transition metal surfaces, such as Pt, Ni, Fe, Co etc [1] . Rh, due to its special application as catalysts in the catalytic or electrochemical reaction has made it one of the most important materials in surface sciences. It will be of great help for understanding the interfacial phenomenon and possibly the SERS mechanisms if we can extend SERS to the Rh surface. However, It has been found that, Rh is very difficult to be roughened since it will grow naturally in air an oxide layer which will retard the further formation of surface oxides. Furthermore, Rh is very easily oxidized to various forms of rhodium oxides as can be found from the phase diagram of Rh [2] . How to select a method to roughen the surface and to extend SERS to Rh seems not an easy task. In our previous study, we found that Rh could not be successfully roughened using controlled?potential roughening procedure as has been used for Pt. In the present study we developed a method to roughen the Rh surface for obtaining strong Raman scattering based on the work of Shibata [3] . It reveals that the etching of Rh is possible by applying a pulse current with a suitable frequency. With the pulse current, we can easily polarize the electrode to a high potential with a high current density, that makes the deep oxidation of Rh surface possible. After the systematic work of our group, we found that an Rh electrode with reasonable good SERS activity could be obtained by applying the current between -30 mA to +50 mA with a frequency ranging between 200 to 800 Hz. The roughened Rh surface presents a quite uniform surface structure. The most important feature is the electrochemical behavior of this kind of electrode is almost the same as that of the smooth surface, see Fig. 1. It can be clearly seen that both cyclic voltammograms present the oxidation of Rh at positive potentials and the hydrogen adsorption/desorption at negative potentials, and at very negative potentials, the hydrogen evolution occurs on the surface. This ensures the Raman spectra obtained is in representative of that from bulk electrodes, which is distinctly different from Ag, Cu and Au surfaces that after roughened in KCl solution, the electrochemical behavior changed dramatically. Using this kind of surface, we selected pyridine as the model molecule to check the applicability of this method for the surface Raman study. The Raman spectra were acquired on a LabRam I spectrometer that has very high detection sensitivity. The solution used was 0.01 mol/L pyridine + 0.1 mol/L NaClO 4. It could be seen from Fig. 2 that the surface Raman signal of pyridine is very strong, with characteristic bands appearing at 1 003 , 1 208 and 1 590 cm -1 respectively. The relative band intensity and the band position is very close to that of Pt electrode while distinctively different from that of noble metal and Fe and Ni electrodes. This indicates a different interaction of pyridine with different transition metal surfaces. This kind of surface shows quite good stability. It is very stable even after a long time exposure in air. Upon very negative or positive potential excursion of the electrode then back to -0.8 V, the pyridine signal reduced to about 20% and 50% that of the freshly prepared Rh surface. One might think that the reversibility of this electrode is not as good as that of the Pt surface. However, before or after each experiment, when it was cleaned in 0.1 mol/L H 2SO 4 solution until reproducible cyclic voltammograms obtained, the pyridine signal can be recovered to about 80% compared with that